(1) Field of the Invention
This invention relates to an improved method of screen gravure photoengraving which can be utilized for the photogravure printing process.
More particularly, the invention relates to an improved screen gravure photoengraving method in which several photosensitive materials and half-tone positives can be used.
Furthermore, the invention relates to an improved screen gravure photoengraving method in which excellent gravure printings with wide gradation and good reproducibility are produced.
(2) Description of Prior Art
In the ordinary gravure photoengraving methods, the conventional gravure method and the screen gravure method are generally known. In the conventional gravure method, the density gradation is represented by the variation of depths of ink cells formed in a printing plate, while in the latter screen gravure method, the density gradation is represented by the variation of sizes of ink cells or by the variation of both the sizes and depths of ink cells.
In the conventional gravure photoengraving process, several kinds of etching solutions are necessary for etching a printing plate through a layer of photosensitive material since the gradation must be represented only by the depths of ink cells formed in the printing plate. The screen gravure method, that is the so-called intaglio half-tone gravure method, is an improvement of the former conventional method, however, when the gradation of an original is represented only by the variation of the sizes of ink cells, the gradation of resultant prints is not satisfactory and the tone of the original picture can not be well reproduced. While, in the method utilizing the combination of both size and depth variations of ink cells, the influence of deviation in depths can be reduced because the factor of cell size is added, so that the latter method is preferable in view of reproducibility.
However, in the known method called as double positive method, though the ink cells are varied in both the sizes and depths, two kinds of positives such as a gravure half-tone positive and a continuous tone positive must be used. That is, in this process, a photosensitive material such as carbon resist is firstly exposed to a light source through a half-tone positive to form half-tone dots, then it is further exposed to the light through a continuous tone positive to give the variation of depths of the half-tone dots, thereafter a printing plate is formed through the steps of transferring, developing and etching. In this process, it is necessary to register completely the half-tone positive with the continuous tone positive, which is substantially difficult, and in addition, the process of exposure is complicated. Accordingly, very high skill is required for this process.